<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>边框流动</title>
    <style>
        .box{
            position: relative;
            width: 180px;
            height: 250px;
            background: rgba(0,0,0,0.8);
            border-radius: 40px;
            /*内容居中*/
            display: flex;
            justify-content: center;
            align-items: center;
        }
        .box h2{
            color: white;
            font-size: 4em;
            text-shadow: 2px 2px pink;
            z-index: 1;
        }
        .box::before{
            content: "";
            position: absolute;
            width: 120px;
            height: 120%;
            background: linear-gradient(#00ccff,#d500f9);
        }
        .box::after{
            content: "";
            position: absolute;
            background: #0e1538;
            inset:5px;
            border-radius:40px;
        }
        .box::before{
            animation: rotate 4s linear infinite;
        }
        @keyframes rotate {
            from{
                transform: rotate(0deg);
            }
            to{
                transform: rotate(360deg);
            }
        }

        .box{
            overflow: hidden;
        }
    </style>
</head>
<body>
<div class="box">
    <h2>CSS</h2>
</div>

</body>
</html>